The tank expanded when the super-cold fuel was drained after Sunday's launch was canceled because of the weather. The ice that formed "pinched" some of that foam, causing the quarter-inch-wide crack and the piece of foam to drop off, Shannon said.

A gopher tortoise moves down the road to the space shuttle Discovery, seen sitting on the launch pad at the Kennedy Space Center in Cape Canaveral, Florida, July 3, 2006. NASA inspectors found a crack in the foam insulation of space shuttle Discovery's fuel tank and managers were meeting on Monday to decide how the crack might affect Tuesday's launch. [Reuters]

The size of the fallen foam was less than half the size of one that could cause damage, NASA officials said.

"Although it is in an area that we don't like to see foam come off ... it would not have caused damage to the orbiter itself," said John Shannon, deputy manager of the space shuttle program.

Outside experts had mixed opinions on what NASA should do.

Osheroff said he was disturbed to hear about the small chunk of foam that fell off while the spacecraft was at rest. His own experiments at Stanford University show that the foam would weigh double what NASA anticipates if water gets into it, leading to potentially catastrophic results if it hits the shuttle.

On the other hand, another member of the Columbia accident board, John Logsdon, director of the Space Policy Institute at George Washington University, said NASA doesn't have "all the information, but they have no information that says not to go."

Inspectors spotted the crack in the foam insulation during an overnight check of the shuttle. NASA had scrubbed launch plans Saturday and Sunday because of weather problems.

The forecast for a Tuesday liftoff was better than previous days, with just a 40 percent chance that storm clouds would prevent liftoff.

NASA Administrator Michael Griffin decided last week that the shuttle should go into orbit as planned, despite the concerns of two top agency managers, including the top safety officer, who wanted additional repairs to the foam insulation.

The mission for Discovery's crew this time is to test shuttle-inspection techniques, deliver supplies to the international space station and drop off European Space Agency astronaut Thomas Reiter for a six-month stay.